1. Field of the Invention
The present invention relates to a catheter for intravascular pressure measurement including an outer and an inner sheath.
2. Discussion of the Related Art
Intravascular pressure measurement involves the measurement of pressure inside a blood vessel or inside an organ to which the blood vessel leads, e.g. the heart. For this purpose, the catheter includes a so-called guide wire; known per se, which is arranged in the sheath and which is used for guiding the catheter through the blood vessel and its branches, and up to the place where pressure measurement is to be performed.
In continuous pressure measurement it is important that the pressure transducer is correctly calibrated. By this is meant that there shall be a definite relationship between the output signal of the transducer and the pressure measured by it. In electronic circuits which transmit measuring signals, signal drift often occurs due to temperature rise in the electrical components, aging of components etc. The output signal from the pressure transducer differs from transducer to transducer. For relating the output signal of the transducer, and the signal amplified by the electronic equipment, to a given pressure, e.g. 1.00 bar, the transducer is calibrated by subjecting it to this pressure of 1.00 bar, the measuring signal then being taken and the value read is then allotted to this pressure. By carrying out a series of calibrations at different pressures there is obtained a so-called calibration graph, and with this as basis the output signal of the transducer can be related to a given, definite, absolute pressure prevailing at the moment.
It is desirable to calibrate the transducer without needing to take it out of the blood vessel. The advantage with such in situ calibration is that the calibration graph is plotted with the transducer at the location of measurement. If, for example, the transducer is calibrated in a laboratory and is then transferred to an operating theatre, and lastly is inserted into the blood vessel, there is a risk that the properties of the transducer become changed due to extraneous mechanical action in the form of blows and knocks. There is also the risk that the lines connected to the transducer are disturbed or damaged. Such damage could result in false pressure measurement values. In situ calibration is also advantageous, since the pressure transducer does not need to be taken out of the blood vessel for calibration and subsequently reinserted in it, which is painful for the patient and is also a time-consuming procedure.
It is known to measure the pressure intravascularly using a pressure transducer which is in communication with the pressure registration apparatus, either via a liquid or an air column. The known apparatus of this kind have slow dynamic response and are sensitive to movement artefacts which limits the reliability of the pressure measurement.
In the Swedish patent documents 8500104-8, 8602836-2 and 8603304-0 there are described a pressure measurement system, a miniaturized sensor and a so-called guide wire.